As you are probably aware a go around is a standard manoeuvre albeit one that is not carried out with any great frequency. It will normally be carried out once or more on every six monthly simulator check. On line it might not occur for years then you have two in a week, such is life. A pilot will carry out a self instigated go around if the flight deck crew are not assured that a safe landing can be made. Examples of such situations are: onboard warnings/cautions; unsecured cabins; fast or high approaches such that time has not conspired to have the aircraft in an acceptable "slot" at 500 ft ; pilot incapacitation; ground lighting or equipment failures, etc. On the other hand ATC will often instigate a go around for example when : Spacing between aircraft becomes degraded; an aircraft taking off fails to comply or hear the clearance; an aircraft taking off aborts its take off roll; a landing aircraft is slow to clear the runway; contamination of the runway by debris, birds or vehicle intrusion has been detected or anticipated, etc.
Given the safety imperative, costs are irrelevant but will amount to around 10 minutes extra flying. Most airlines will not expect any explanation from the crew and even if one is given or requested then so be it. That said, passengers and cabin crew are not expecting it to happen and since some might find it alarming, then if time permits a quick P.A might be worthy of some consideration.